The use of H2 as a potential source of clean and renewable fuel has attracted great interest in an effort to reduce current dependence on fossil fuels.[1] Reduction of water to H2, especially with visible light, has been a subject of intense study and a significant amount of efforts have been devoted towards designing metal complexes for proton reduction.
Over the past few years, a number of H2 evolution catalysts based on metal complexes such as Co,[2] Ni,[3] Fe,[4] and Mo[5] have been reported and studied, especially in nonaqueous media, to provide insights into the mechanism of proton reduction. Recently, Eisenberg and coworkers described photocatalytic proton reduction catalyzed by a mononuclear cobalt-dithiolene complex with remarkable turnover number (TON) of >2700 per mol of Co catalyst in a 1:1 ratio of CH3CN/H2O.[2h]
Although there has been significant progress in designing molecular catalysts for H2 evolution, the search of robust and highly active catalysts that can operate in purely aqueous solution, by either electrochemical or photochemical approaches, still remains a great challenge.[2e, 6]